1. Field of the Invention
The present invention relates to a superconducting cable, and more particularly to a compact superconducting power transmission cable operating at distribution level voltages.
2. Description of the Prior Art
A superconducting power transmission cable must be capable of transmitting a heavy current with low energy loss in a compact conductor. Power transmission is generally made through an alternating current, and a superconductor employed under an alternating current is inevitably accompanied by an energy loss, which is generically called an ac loss.
A previous patent describing a superconducting power cable is found in U.S. Pat. No. 5,932,523 to Fujikami et al. Fujikami describes a superconducting cable conductor employing an oxide superconductor, which comprises a long flexible core member, a plurality of tape-shaped multifilamentary oxide superconducting wires, which are spirally wound on the core member, and an electric insulating layer. The plurality of tape-shaped superconducting wires wound on the core member form a plurality of layers, each of which is formed by winding a plurality of tape-shaped superconducting wires in a side-by-side manner. The plurality of layers are successively stacked on the core member.
U.S. Pat. No. 5,200,577 to Shimada describes a large electric capacity superconducting cable that is formed by twisting a bundle of a plurality of superconducting material wires with each other about a core member arranged at a central portion of the bundle.
U.S. Pat. No. 4,409,425 to Ries describes how the stabilization of the superconducting elements is achieved by arranging further special stabilizing elements of normal conducting material parallel to the superconducting elements. The stabilizing elements and the superconducting elements are twisted together to form a flat cable and can be arranged around a carried body in a ribbon shape.
There are a number of patents describing superconducting wires composed primarily of a bismuth-strontium-calcium-copper-oxide (BSCCO). The methods of forming the wires are described in U.S. Pat. No. 5,330,969 to Finnemore et al and U.S. Pat. No. 6,069,116 to Li et al. By using this material a superconducting power transmission cable can be made such that it operates at distribution level voltages. There is interest in the electric power utility industry for a design of a 3-phase cable, in 100 MVA, with a diameter of less than 100 mm, operating at distribution level voltages, e.g. 30 kV.
An object of the present invention is to provide a manufacturable compact superconducting power transmission cable capable of operating at distribution level voltages. This design positions the superconducting material to minimize the magnetic field component perpendicular to the conductor face, thus reducing the ac losses of the superconductor. This allows the conductors within a phase of the cable, and the separate phases of a three-phase cable, to be placed closer together than in existing designs. This results in a very compact cable for a given power rating.
The three phase cable is made of up of three phase conductors surrounded by a dielectric. The whole cable is surrounded by thermal insulation. Each phase conductor of a three phase cable is made of up of a number of parallel subconductors and has a void space for liquid coolant.
Each subconductor is a planar array of a number of High Temperature Superconductors (HTS). The spacing between the HTS conductors within each array is chosen to reduce the perpendicular component of magnetic field on the two edge conductors due to the current in the HTS conductors. The number of subconductors per phase and the number of HTS conductors per subconductors are chosen to minimize the overall diameter of the cable within the constraints of the allowed ac losses of the cable.
A preferred form of the superconducting power transmission cable, as well as other embodiments, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read with in connection with the accompanying drawings.